1. Field of the Invention
This invention relates to seat suspensions for vehicles, and to vehicle seats incorporating such suspensions.
2. Description of the Prior Art
It is well-known to provide a vehicle seat, in particular in a heavy goods vehicle, of the type in which a seat part is mounted on a base part by means of a spring suspension which isolates the seat part from at least some of the vibration of the vehicle cab on which the base part of the seat is mounted. The spring in such a suspension can be a mechanical spring, or a spring containing a charge of compressed air or other gas. It is also known to use such an air spring to vary the height of the seat part by providing a valve connecting the interior of the air spring to a source of compressed air. The valve can be opened manually to allow compressed air to enter the spring to counterbalance the weight of the seat occupant and to extend the air spring to a selected extent so that the seat occupant is located at the height he chooses. Another manually operable valve is provided to exhaust air from the air spring. An example of such a seat is disclosed in British Pat. No. 1,119,902 in which manual operation of a lever causes the seat to be raised or lowered automatically to a selected or preselected ride height.
In order however for such a system to be fully effective it is desirable that it should operate automatically, for example upon the seat occupant sitting on the seat. If it is left to the seat occupant to switch it on at will, he is liable to fail to do so and drive the vehicle with the seat at an incorrect height. If however the device is automatically switched on for example by switching on the vehicle's ignition circuit, or by a seat-operated switch responsive to load on the seat, so that it remains switched on while the seat is occupied, movement of the vehicle over rough terrain will produce the same response as repeatedly changing the weight of the seat occupant, and the seat will repeatedly rise and fall.